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FAQ
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Frequently Asked Questions (FAQ) for the UCD/Net-SNMP package
=============================================================
FAQ Author: Dave Shield
Net-SNMP Version: 5.8.dev
Net-SNMP/UCD-SNMP Project Leader: Wes Hardaker
Email: [email protected]
TABLE OF CONTENTS
=================
TABLE OF CONTENTS
GENERAL
What is it?
Where can I get it?
What documentation is available?
Are there binaries available?
What's the difference between UCD-SNMP and Net-SNMP?
What operating systems does it run on?
What happens if mine isn't listed?
Does it run on Windows?
How do I find out about new releases?
How can I find out what other people are doing?
How do I submit a patch or bug report?
Can I reuse the code in my commercial application?
What's the difference between SNMPv1, SNMPv2 and SNMPv3?
What's the difference between SNMPv2 and SNMPv2c?
Which versions of SNMP are supported in this package?
Can I use SNMPv1 requests with an SNMPv2 MIB (or vice versa)?
How can I monitor my system with SNMP?
Where can I find more information about network management?
What ports does SNMP use?
Is Net-SNMP thread safe?
APPLICATIONS
How do I add a MIB?
How do I add a MIB to the tools?
Why can't I see anything from the agent?
Why doesn't the agent respond?
I can see the system group, but nothing else. Why?
Why can't I see values in the <ENTERPRISE> tree?
The agent worked for a while, then stopped responding. Why?
Requesting an object fails with "Unknown Object Identifier" Why?
Why do I get "noSuchName" when asking for "sysUpTime" (or similar)?
Why do I sometimes get "End of MIB" when walking a tree, and sometimes not?
How do I use SNMPv3?
Why can't I set any variables in the MIB?
Variables seem to disappear when I try to set them. Why?
Why can't I change sysLocation (or sysContact)?
I get an error when trying to set a negative value - why?
I get an error when trying to query a string-indexed table value - why?
How should I specify string-indexed table values?
How do I send traps and notifications?
How do I receive traps and notifications?
How do I receive SNMPv1 traps?
Why don't I receive incoming traps?
My traphandler script doesn't work when run like this - why not?
How can the agent receive traps and notifications?
How big can an SNMP request (or reply) be?
How can I monitor my systems (disk, memory, etc)?
Applications complain about entries in your example 'snmp.conf' file. Why?
OK, what should I put in snmp.conf?
How do I specify IPv6 addresses in tools command line arguments?
PERL
What is the purpose of the Perl SNMP module?
Where can I get the Perl SNMP package?
How do I install the Perl SNMP modules?
But compiling this fails! Why?
Compiling the Perl module works OK, but 'make test' fails. Why?
Why can't mib2c (or tkmib) locate SNMP.pm?
Why can't mib2c (or tkmib) load SNMP.so?
Why can't tkmib locate Tk.pm?
Why does your RPM complain about missing Perl modules?
I've got a problem with the Net-SNMP module. Can you help?
MIBS
Where can I find a MIB compiler?
Why aren't my MIB files being read in?
Where should I put my MIB files?
What does "Cannot find module (XXX-MIB)" mean?
I'm getting answers, but they're all numbers. Why?
What does "unlinked OID" mean?
The parser doesn't handle comments properly. Why not?
How can I get more information about problems with MIB files?
What's this about "too many imported symbols"?
Do I actually need the MIB files?
AGENT
What MIBs are supported?
What protocols are supported?
How do I configure the agent?
How do I remove a MIB from the agent?
I've installed a new MIB file. Why can't I query it?
How do I add a MIB to the agent?
What's the difference between 'exec', 'sh', 'extend' and 'pass'?
What's the difference between AgentX, SMUX and proxied SNMP?
What is the purpose of 'dlmod'?
Which should I use?
Can I use AgentX when running under Windows?
How can I run AgentX with a different socket address?
How can I turn off SMUX support?
How can I combine two copies of the 'mib2' tree from separate subagents?
What traps are sent by the agent?
Where are these traps sent to?
How can I send a particular trap to selected destinations?
When I run the agent it runs and then quits without staying around. Why?
After a while the agent stops responding, and starts eating CPU time. Why?
How can I stop other people getting at my agent?
How can I listen on just one particular interface?
The agent is complaining about 'snmpd.conf'. Where is this?
Why does the agent complain about 'no access control information'?
How do I configure access control?
How do I configure SNMPv3 users?
The 'createUser' line disappears when I start the agent. Why?
What's the difference between /var/net-snmp and /usr/local/share/snmp?
My new agent is ignoring the old snmpd.conf file. Why?
Where should the snmpd.conf file go?
Why am I getting "Connection refused"?
Why can't I see values in the UCDavis 'extensible' or 'disk' trees?
Why can't I see values in the UCDavis 'memory' or 'vmstat' tree?
What do the CPU statistics mean - is this the load average?
How do I get percentage CPU utilization using ssCpuRawIdle?
What about multi-processor systems?
The speed/type of my network interfaces is wrong - how can I fix it?
The interface statistics for my subinterfaces are all zero - why?
Does the agent support the RMON-MIB?
What does "klread: bad address" mean?
What does "nlist err: wombat not found" (or similar) mean?
What does "Can't open /dev/kmem" mean?
The system uptime (sysUpTime) returned is wrong!
Can the agent run multi-threaded?
Can I use AgentX (or an embedded SNMP agent) in a threaded application?
COMPILING
How do I control the environment used to compile the software?
How do I control the environment used to compile the software under Windows?
Why does the compilation complain about missing libraries?
How can I reduce the memory footprint?
How can I reduce the installation footprint or speed up compilation?
How can I compile the project for use on an embedded system?
How can I compile the project to use static linking?
Why does 'make test' skip various tests?
Why does 'make test' complain about a pid file?
CODING
How do I write C code to integrate with the agent?
How does the agent fetch the value of a MIB variable from the system?
Mib2c complains about a missing "mib reference" - what does this mean?
Mib2c complains about not having a "valid OID" - what does this mean?
Why doesn't mib2c like the MIB file I'm giving it?
Mib2c ignores my MIB and generates a pair of 'mib-2' code files. Why?
What's the difference between the various mib2c configuration files?
Which mib2c configuration file should I use?
How can I have mib2c generate code for both scalars and tables?
Are there any examples, or documentation for developing MIB modules?
Where should I put the files produced by 'mib2c'?
Why doesn't my new MIB module report anything?
Why does the iterator call my get_{first,next} routines so often?
How can I get the agent to generate a trap (or inform)?
How can I get an AgentX sub-agent to generate a trap (or inform)?
How can I get the agent to send an SNMPv1 (or SNMPv2c) trap?
How can I get the agent to include varbinds with an SNMPv1 trap?
How can I get the agent to send an SNMPv1 enterprise-specific trap?
How can I get the agent to send an SNMPv3 trap (or inform)?
Why does calling 'send_v2trap' generate an SNMPv1 trap (or vice versa)?
How can I register a MIB module in a different (SNMPv3) context?
MISC
What ASN.1 parser is used?
What is the Official Slogan of the net-snmp-coders list?
GENERAL
=======
What is it?
----------
- Various tools relating to the Simple Network Management Protocol
including:
* An extensible agent
* An SNMP library
* tools to request or set information from SNMP agents
* tools to generate and handle SNMP traps
* a version of the unix 'netstat' command using SNMP
* a graphical Perl/Tk/SNMP based mib browser
This package is originally based on the Carnegie Mellon University
SNMP implementation (version 2.1.2.1), but has developed significantly
since then.
Where can I get it?
------------------
Download:
- http://www.net-snmp.org/download/
- ftp://ftp.net-snmp.org/pub/sourceforge/net-snmp/
Web page:
- http://www.net-snmp.org/
Sourceforge Project page:
- http://www.net-snmp.org/project/
Mirrors (note that sourceforge download servers are mirrored themselves):
- US: ftp://ftp.freesnmp.com/mirrors/net-snmp/
- Greece: ftp://ftp.ntua.gr/pub/net/snmp/net-snmp/
What documentation is available?
-------------------------------
This FAQ (!)
README and individual READMEs for various platforms
README.thread (discusses threading issues)
INSTALL
PORTING
EXAMPLE.conf
man pages for the individual tools, files and the API
A guide for extending the agent
Tutorials for both ucd-snmp v4 and net-snmp v5
at http://www.net-snmp.org/tutorial/
and http://www.net-snmp.org/tutorial-5/ respectively
Most of this documentation (plus archives of the mailing lists)
is also available on our web page:
http://www.net-snmp.org/
There is also a Wiki (including a community-maintained version
of this FAQ) at
http://www.net-snmp.org/wiki/
Are there binaries available?
----------------------------
There are binaries for some versions/systems available under
the "net-snmp binaries" package on the SourceForge "Files"
page, which is linked to from the main project download web
page at http://www.net-snmp.org/download.html.
These binaries are also available on the project FTP site,
with a link on the same web page.
There is also a mirror at ftp://ftp.freesnmp.org/mirrors/net-snmp/
What's the difference between UCD-SNMP and Net-SNMP?
---------------------------------------------------
Not a great deal, really.
Although the project originally started at UC Davis (hence the name),
and it has always been based there, most of the contributors have had
little or no connection with this institution.
The move to SourceForge was intended to provide a more flexible
environment for the project, and to distribute the administrative
workload more evenly. The change of name simply reflects this move,
which was the last remaining link with UC Davis.
The 4.2.x line saw the last releases made using the ucd-snmp name,
and all releases on this line have been been bug-fixes only. Release
5.0 was the first version released under the Net-SNMP name, and all
further development is being done on the 5.x code base. The 4.2.x
code line is now effectively closed down, as are the older 5.x branches.
Much of the work done for the various 5.x releases has involved
some fairly significant changes to the code - in particular the
architecture of the agent. However attempts have been made to retain
backwards compatibility as much as possible, and most code written
for earlier releases should continue to work. The most visible
change from the 4.2.x UCD suite to the 5.x Net-SNMP releases was a
restructuring of the header file organisation - not least a change
from <ucd-snmp/xxx.h> to <net-snmp/yyy.h>.
But given the maturity of the Net-SNMP code, this should be less
of a consideration for most current SNMP development projects.
What operating systems does it run on?
-------------------------------------
Both the applications and the agent have been reported as running
(at least in part) on the following operating systems:
* Linux (kernels 2.6 to 1.3)
* Solaris/SPARC (11 to 2.3), Solaris/Intel (10, 9) -- see
README.solaris
* HP-UX (11.31 to 9.01) -- see README.hpux11
* Mac OS X (10.5 to 10.1) -- see README.osX
* NetBSD (2.0 to 1.0)
* FreeBSD (7.0 to 2.2)
* OpenBSD (4.0 to 2.6)
* BSDi (4.0.1 to 2.1)
* AIX (6.1, 5.3, 5.2, 5.1, 4.3.3, 4.1.5, 3.2.5) -- see README.aix
* IRIX (6.5 to 5.1)
* OSF (4.0, 3.2 and Tru64 Unix 5.1B) -- see README.tru64
* SunOS 4 (4.1.4 to 4.1.2)
* Ultrix (4.5 to 4.2)
* Dynix/PTX 4.4
* QNX 6.2.1A
We have also been informed about a port to the Stratus VOS.
See http://ftp.stratus.com/vos/network/network.html for details.
See the next question but one for the status of Windows support.
Certain systems fail to compile particular portions of the agent.
These can usually be persuaded to compile (at the loss of some
functionality) by omitting the modules affected.
See the next question for more details.
Also note that the presence of a particular configuration in this
list does not imply a perfect or complete implementation. This
is simply what various people have reported as seeming to work.
(Or more frequently, the configurations where people have reported
problems that we think we've subsequently fixed!)
What happens if mine isn't listed?
---------------------------------
It's probably worth trying to compile it anyway. Unless your
system is significantly different to the supported configurations,
most of the code (library, applications and the agent infrastructure)
should probably compile with little or no difficulty. The most
likely source of problems will be MIB modules within the agent,
as this tends to be where the most system-specific code is found.
If only a few modules fail to compile, try removing them from
the agent by running "configure --with-out-mib-module=xxx,yyy",
and re-compiling. If a large number of modules fail, then it
might be easier to start from a relatively bare system, using
"configure --enable-mini-agent --with-defaults". Then if this
minimal agent compiles and runs successfully, try adding each of
the missing mibgroups individually using the configure option
'--with-mib-module'.
If configure fails with "invalid configuration" messages, or
you get completely stuck, contact the coders list for advice.
Similarly, if you manage to get this working on a new system,
please let us know of any code changes that you needed to make,
together with details of the hardware you're using, and what
versions of the operating system you've tried it on. The entry
'host' in the file 'config.status' should show this information.
Oh, and congratulations!
Does it run on Windows?
----------------------
The suite should compile and run on Win32 platforms, including
the library, command-line tools and the basic agent framework.
Note that the agent now includes support for the MIB-II module,
but this requires Microsoft's Core Platform SDK. Instructions
for how to install this are given in README.win32.
Pre-compiled binaries are available from the project web site.
As of v5.4, the Net-SNMP agent is able to load the Windows SNMP
service extension DLLs by using the Net-SNMP winExtDLL extension.
Some other Net-SNMP MIB modules, including the UCD pass-through
extensions, do not currently work under Windows. Volunteers to assist
with these missing modules are likely to welcomed with open arms :-)
Further details of Windows support (currently Visual C++, MinGW
and Cygnus cygwin32) is available in the file README.win32.
How do I find out about new releases?
------------------------------------
There is a mailing list for these announcements
To be added to (or removed from) this list, visit
http://www.net-snmp.org/lists/net-snmp-announce/
Or you can send a message to the address
with a subject line of 'subscribe' (or 'unsubscribe' as appropriate).
Advance notice of upcoming releases are also made on the
net-snmp-users list (for "release candidates") for a week
or two before the full release, and on the net-snmp-coders
list (for "pre-releases") during the period prior to this.
Major code revisions may be announced more widely, but these
lists are the most reliable way to keep in touch with the
status of the package.
Patches to fix known problems are also made available via the web site:
http://www.net-snmp.org/patches/
How can I find out what other people are doing?
----------------------------------------------
There is a general purpose discussion list
To be added to (or removed from) this list, visit
http://www.net-snmp.org/lists/net-snmp-users/
Or you can send a message to the address
with a subject line of 'subscribe' (or 'unsubscribe' as appropriate).
To find out what the developers are doing, and to help them
out, please read the PORTING file enclosed with the package.
There is also a #net-snmp IRC channel set up on the freenode.net
chat system. You can connect to this via chat.freenode.net.
See http://www.freenode.net/ for more information on getting
started with IRC.
Several core developers hang out on this channel on a fairly
regular basis.
How do I submit a patch or bug report?
-------------------------------------
The best way to submit a bug report is via the bug database through
the interface found at
http://www.net-snmp.org/bugs/
Be sure to include the version of the package that you've been working
with, the output of the command 'uname -a', the precise configuration
or command that triggers the problem and a copy of any output produced.
Questions about using the package should be directed at the
[email protected] mailing list. Note that this
mailing list is relatively busy, and the people answering these
questions are doing so out of the goodness of their hearts, and in
addition to their main employment. Please note the following:
- use plain text mail, rather than HTML
- don't resend questions more than once
(even if no-one answered immediately)
- include full details of exact commands and error messages
("I've tried everything, and it doesn't work" isn't much use!)
- do *NOT* send messages to -users and -coders mailing lists
(most developers read both anyway)
- don't mail the developers privately - keep everything on the list
We can't promise to be able to solve all problems, but we'll
certainly try and help. But remember that this is basically an
unsupported package. It's Open Source, so if you need something
fixing badly enough, fundamentally it's up to you to do the work.
All patches should be submitted to the patch manager at
http://www.net-snmp.org/patches/
If possible, submit a bug report describing the patch as well
(referencing it by its patch number) since the patch manager
doesn't contain a decent description field.
The best way to submit patch (diff) information is by checking out
the current code from the development git trunk, making your changes
and then running "git diff" or "git format-patch" after you're done.
(Please see http://www.net-snmp.org/wiki/index.php/Git for further
information on using git with the Net-SNMP project)
If you're working from a source code distribution, and comparing old
and new versions of a code file, use "diff -u OLDFILE NEWFILE"
Can I reuse the code in my commercial application?
-------------------------------------------------
The details of the COPYRIGHTs on the package can be found in the COPYING
file. You should have your lawyer read this file if you wish to use the
code in your commercial application. We will not summarize here what is
in the file, as we're not lawyers and are unqualified to do so.
What's the difference between SNMPv1, SNMPv2 and SNMPv3?
-------------------------------------------------------
What's the difference between SNMPv2 and SNMPv2c?
------------------------------------------------
A full description is probably beyond the scope of this FAQ.
Very briefly, the original protocol and admin framework was
described in RFCs 1155-1157, and is now known as SNMPv1.
Practical experience showed up various problems and deficiencies
with this, and a number of revised frameworks were developed to try
and address these problems. Unfortunately, it proved difficult to
achieve any sort of agreement - particularly over the details of
the administrative framework to use.
There was less disagreement over the proposed changes to the
protocol operations. These included:
* increasing the range of errors that could be reported
* introducing "exception values"
(so a single missing value didn't affect
the other varbinds in the same request)
* a new GETBULK operation
(a supercharged GETNEXT)
* new notification PDUs
(closer in structure to the other request PDUs)
Strictly speaking, it's this revised protocol (originally defined
in RFC 1905, and most recently in RFC 3416) that is "SNMPv2".
The only framework based on this protocol that saw a significant
level of use was "Community-based SNMPv2" or "SNMPv2c" (defined
in RFC 1901). This retained the same administrative framework
as SNMPv1 (with all of the accompanying limitations), but using
the new protocol operations.
More recently, a new administrative framework has been developed,
building on the various competing SNMPv2 proposals, and using the
same SNMPv2 protocol operations. This is SNMPv3, which is defined
in RFCs 3411-3418. It addresses some of the deficiencies of the
community-based versions, including significant improvements to
the security of SNMP requests (like it finally has some!).
SNMPv3 is now a full IETF standard protocol.
Strictly speaking, SNMPv3 just defines a fairly abstract framework,
based around the idea of "Security Models" and "Access Control Models".
It's this combination of SNMPv3 plus accompanying models that actually
provides a working SNMP system.
However, the only models in common use are the "User-based Security
Model" (RFC 3414) and the "View-based Access Control Model" (RFC 3415).
So "SNMPv3" is frequently used to mean the combination of the basic
SNMPv3 framework with these two particular models.
This is also sometimes described as "SNMPv3/USM".
So in brief:
- SNMPv2c updated the protocol operations
but left the administrative framework unchanged.
- SNMPv3 updated the administrative framework
but left the protocol operations unchanged.
Which versions of SNMP are supported in this package?
----------------------------------------------------
This package currently supports the original SNMPv1 (RFC 1157),
Community-based SNMPv2 (RFCs 1901-1908), and SNMPv3 (RFCs 3411-3418).
The agent will respond to requests using any of these protocols,
and all the tools take a command-line option to determine which
version to use.
Support for SNMPv2 classic (a.k.a. "SNMPv2 historic" - RFCs 1441-1452)
was dropped with the 4.0 release of the UCD-snmp package.
Can I use SNMPv1 requests with an SNMPv2 MIB (or vice versa)?
------------------------------------------------------------
Yes.
The syntax used to specify a MIB file (better referred
to as SMIv1 or SMIv2) is purely concerned with how to define
the characteristics of various management objects. This is
(almost) completely unrelated to the versions of the protocol
used to operate on these values. So it is quite reasonable to
use SNMPv1 requests on objects defined using SMIv2, or SNMPv2
(or SNMPv3) requests on objects defined using SMIv1.
The one exception is objects of syntax Counter64, which are
only accessible using SNMPv2 or higher. SNMPv1 requests will
either treat such objects as an error, or skip them completely.
Note that SMIv1 is effectively obsolete, and all new MIBs
should be written using SMIv2.
How can I monitor my system with SNMP?
-------------------------------------
There are two main methods of using SNMP for monitoring. One is to regularly
query the SNMP agent for information of interest, graphing these values and/or
saving them for later analysis. That's not really the focus of the Net-SNMP
project - our tools are more low-level, single-shot commands. For this sort
of high-level management, you're really looking at a management console
application (such as Nagios or OpenNMS), or a data logging application
(such as RRDtool, or one of its front-ends - MRTG, Cacti, etc).
The other approach is to configure the SNMP agent to monitor the relevant
information itself, and issue an alert when the values pass suitable limits.
See the section ACTIVE MONITORING in the snmpd.conf(5) man page for details.
Note that this entry makes no reference as to _what_ you should monitor, or
what values might be significant. That's because it is impossible to provide
a universal answer to these questions. The information to monitor, and the
normal operating values will ultimately depend on your local environment.
SNMP is simply a tool to _help_ you manage your systems - it isn't a magic
panacea - you still have to think for yourself!
Where can I find more information about network management?
----------------------------------------------------------
There are a number of sites with network management information on
the World Wide Web. Some of the most useful are
http://www.simpleweb.org/
http://www.snmplink.org/
http://www.mibdepot.com/
The SNMP Usenet newsgroup is now mostly defunct, but although the
FAQ hasn't been updated for a while, it still contains a large
amount of useful information relating to SNMP, including books,
software, other sites, how to get an enterprise number, etc, etc.
This is available from
ftp://rtfm.mit.edu/pub/usenet/comp.protocols.snmp/
or via any of the Web sites above.
What ports does SNMP use?
------------------------
There are three main network ports (and one named socket), which are
typically used by SNMP. These are:
- UDP port 161 - SNMP requests (GET* and SET)
- UDP port 162 - SNMP notifications (Traps/Informs)
- TCP port 705 - AgentX
- /var/agentx/master - AgentX
However, these are simply the default "well-known" ports for these purposes,
and it is perfectly possible to accept requests on other ports.
Is Net-SNMP thread safe?
-----------------------
Strictly speaking, no. However, it is possible to use the library within
a multi-threaded management application. This is covered in detail in
the file README.thread (shipped with the standard distribution), but can
be summarised as follows:
- Call 'snmp_sess_init()' prior to activating any threads.
This reads in and parses MIB information (which isn't thread-safe)
as well as preparing a session structure for subsequent use.
- Open an SNMP session using 'snmp_sess_open()' which returns an
opaque session handle, which is essentially independent of any
other sessions (regardless of thread).
- Resource locking is not handled within the library, and is the
responsibility of the main application.
The Net-SNMP agent has not been designed for multi-threaded use. It
should be safe to use the agent library to embed a subagent within a
threaded application as long as *all* SNMP-related activity (including
generating traps, and parsing MIBs) is handled within a single thread.
The command-line tools shipped as part of the Net-SNMP distribution
are simple single-threaded applications, and are not designed for
multi-threaded use. Adapting these to a threaded model is left as
an exercise for the student.
The same holds true for the notification receiver (snmptrapd).
Unfortunately, the SNMPv3 support was added about the same time as
the thread support and since they occurred in parallel the SNMPv3
support was never checked for multi-threading correctness. It is
most likely that it is not thread-safe at this time.
APPLICATIONS
============
How do I add a MIB?
------------------
This is actually two separate questions, depending on whether you
are referring to the tools, or the agent (or both).
See the next question or the next section respectively.
How do I add a MIB to the tools?
-------------------------------
Adding a MIB to the client-side tools has two main effects:
- it allows you to refer to MIB objects by name
(rather than having to use the numeric OIDs)
- it allows the results to be displayed in a more immediately
meaningful fashion. Not just giving the object names, but
also showing named enumeration values, and interpreting table
indexes properly (particularly for string and OID index values).
There are two steps required to add a new MIB file to the tools.
Firstly, copy the MIB file into the appropriate location:
cp MY-MIB.txt /usr/local/share/snmp/mibs
(which makes it available to everyone on the system)
or
mkdir $HOME/.snmp
mkdir $HOME/.snmp/mibs
cp MY-MIB.txt $HOME/.snmp/mibs
(which makes it available to you only)
Note that the location of the shared MIB directory may be different
from that given here - see the FAQ entry "Where should I put my MIB
files?" for more information.
Secondly, tell the tools to load this MIB:
snmpwalk -m +MY-MIB .....
(load it for this command only)
or
export MIBS=+MY-MIB
(load it for this session only)
or
echo "mibs +MY-MIB" >> $HOME/.snmp/snmp.conf
(load it every time)
Note that the value for this variable is the name of the MIB
module, *not* the name of the MIB file. These are typically the
same (apart from the .txt suffix), but if in doubt, check the contents
of the file. The value to use is the token immediately before the
word DEFINITIONS at the start of the file.
Or use the special value "all" to have the tools load all available
MIBs (which may slow them down, particularly if you have a large
number of MIB files.
Note that you need *both* steps.
Adding a MIB in this way does *not* mean that the agent will
automatically return values from this MIB. The agent needs to be
explicitly extended to support the new MIB objects, which typically
involves writing new code.
See the AGENT section for details.
Most of the tools (apart from 'snmptable') will work quite happily
without any MIB files at all - although the results won't be displayed
in quite the same way. Similarly, the agent doesn't need MIB files
either (other than to handle MIB object names in the configuration file).
Why can't I see anything from the agent?
---------------------------------------
Fundamentally, there are two basic reasons why a request may go
unanswered. Either the management application does not like the
request (so never sends it), or the agent does not like the request
(so never responds). The simplest way to distinguish between the
two is to run the command with the command-line option '-d'.
If this doesn't display a hex dump of the raw outgoing packet, then
it's the client side which is dropping the request. Hopefully you
should also see an error message, to help identify what's wrong.
If this displays one or more outgoing dumps (but nothing coming back),
then the request is failing at the agent end. See the next entry for
more details.
There are three further possibilities to consider:
One is that the agent may return a response to the original query,
but the management application may not like this response, and refuse
to display it. This is relatively unusual, and typically indicates
a flaw with the remote agent. (I hope you're not contemplating the
suggestion that the Net-SNMP command-line tools might contain bugs!)
The typical symptoms of this would be that the '-d' option would
display a sequence of sending and received packet dumps, with the
same contents each time. Ask on the mailing list for advice.
Alternatively, the agent may simply not support the MIB objects being
requested. This is most commonly seen when using the "snmpwalk" tool
(particularly with SNMPv1).
The symptoms here would be that '-d' would show two pairs of raw
packet dumps - one a GETNEXT request (A1 in the sending packet),
followed by a GET request (A0). Repeating the same request with the
"snmpgetnext" command-line tool should show the information (if any)
that the agent returned, which was then discarded by snmpwalk as
irrelevant.
Note that this is how snmpwalk was designed to work. It is not an error.
Finally, it may be that the agent is simply taking too long to respond.
The easiest way to test for this is to add the command-line options
"-t 60 -r 0", which will send a single request (with no repetitions)
and wait for a minute before giving up. This ought to be long enough
for all but the most-overloaded agent, or inefficient MIB module!
If this turns out to be the cause, then ask on the mailing list for
advice on options for improving the performance.
Why doesn't the agent respond?
-----------------------------
Assuming that the tests outlined in the previous entry indicate that
the problem lies with the agent not responding, the obvious question
is "why not".
Again, there are two basic possibilities - either the agent never
sees the request, or it receives it but is unwilling (or unable) to
process it. If the remote system is running the Net-SNMP agent,
then the easiest way to distinguish between these two cases is to
shut down the agent, and re-start it manually using the options
-f -Le -d
Then send the same query as before. This should display raw dumps of
packets seen (or sent) by the agent, just as with the client side in
the previous entry.
If the agent does not display anything, then it is simply not receiving
the requests. This may be because they are being blocked by network
or local firewall settings ('iptables -L'), or the agent may not be
listening on the expected interfaces ('netstat -a').
This is most commonly encountered when running queries from a remote
host, particularly if the same request succeeds when run on the same
system as the agent itself.
If the agent displays a dump of the incoming request, but nothing going
out, then the most likely cause is access control settings. See the
relevant entries in the AGENT section for details. Note that if the agent
receives an SNMPv1 or SNMPv2c request with a unknown community string,
then it will not return an error response - the request is simply discarded.
Another possibility is that the request may be rejected by settings in
/etc/hosts.{allow,deny}. Again, '-d' will display an incoming packet
dump but no corresponding outgoing response. However in this situation,
the agent should also log a message that the request is being refused.
Running the agent with '-d' can also help identify situations where the
agent *is* responding to the request, but only after a long delay. This
would be indicated by a series of incoming packet dumps (showing various
retries from the client side), followed by several outgoing dumps - possibly
long after the client tool has given up in disgust.
See the entry
The agent worked for a while, then stopped responding. Why?
later in this section.
I can see the system group, but nothing else. Why?
--------------------------------------------------
This is almost definitely due to the access configuration of the agent.
Many pre-configured systems (such as most Linux distributions) will only
allow access to the system group by default, and need to be configured
to enable more general access.
The easiest way to test this is to try a GETNEXT request on one of
the other standard groups
e.g.
snmpgetnext ..... interfaces
If the agent responds with "hrSystemUptime.0" or "end of MIB", then it
is clearly configured in this way. See the entries on access control
in the AGENT section for more information.
Why can't I see values in the <ENTERPRISE> tree?
-----------------------------------------------
If you can see most of the standard information (not just the system and
hrSystem groups), but not in the vendor-specific 'enterprises' tree, then
once again there are several possible causes.
Firstly, it's possible that the agent does not implement this particular
enterprise tree. Remember that adding a MIB to the client tools does
*not* automatically add support for these object to the agent. See the
AGENT section for more information.
Alternatively, it may be that the agent does implement some or all of this
enterprise tree, but the access control settings are configured to block
access to it.
The simplest way to checks whether the agent implements a given portion
of the OID tree is to run
snmpwalk .... nsModuleName
and look for index values that fall in the area of interest.
(Always assuming that you have access to this particular section
of the Net-SNMP enterprise tree, of course!)
Checking the access control settings can be done by examining the tables
vacmAccessTable and vacmViewTreeFamilyTable. Note that these are used
to configure access control for *all* versions of SNMP - not just SNMPv3.
The third possibility is that simply isn't any information in the specified
tree. For example, several of the tables in the UCDavis enterprise tree
(such as prTable, extTable, dskTable and fileTable) require explicit
configuration in the snmpd.conf file. If you query this particular tables
without the necessary configuration entries, then they will be empty.
Finally, if you can't see anything from *any* enterprise-specific tree,
then this may be down to how you are asking for the information. By
default, if "snmpwalk" is run without an explicitly starting OID, then
it will display the contents of the 'mib-2' tree, containing most of the
IETF-standard management information supported by the agent.
When the agent reaches the end of this tree, it will return the first
enterprise-specific value, 'snmpwalk' will recognise that this marks the
end of the (implicit) requested tree, and stop. No enterprise-specific
information will be displayed.
To walk the whole tree, and see *all* the information that the
agent supports, specify a starting point of '.iso' or '.1'.
To walk a specific enterprise subtree, specify the root of this tree
as the starting point - e.g:
snmpwalk -v1 -c public localhost UCD-SNMP-MIB::ucdavis
There is more information about particular UCD-specific subtrees in
the AGENT section.
The agent worked for a while, then stopped responding. Why?
-----------------------------------------------------------
There are three basic possibilities:
- the agent has crashed
- it is hanging
- it is temporarily overloaded
Detecting whether the agent has crashed should be fairly straighforward.
If you can reliably reproduce this crash (e.g. by sending a particular
SNMP request), then contact the coders list for advice.
It's the other two cases that are probably more significant.
To tell the difference between these two, try leaving the agent
undisturbed for a while, and then probe it using a single 'snmpget'
request, specifying a longer timeout (e.g. '-t 120'). If it now
responds, then something was probably sending requests (including
duplicate retries) faster than the agent could process them, and it
was building up a backlog. Try adjusting the timeout period and retry
frequency of these client requests, or look at improving the efficiency
of the implementation of the relevant MIB objects.
If the agent remains unresponsive (particularly if the load on the
system is steadily climbing), then it's probably hanging, and all
you can really do is restart the agent. If you can identify what
causes this to happen, then contact the coders list for advice.
Requesting an object fails with "Unknown Object Identifier" Why?
----------------------------------------------------------------
If a general snmpwalk shows a particular entry, but asking for it more
specifically gives a "sub-identifier not found:" or "Unknown Object
Identifier" error, then that's a problem with the tool, rather than
the agent.
Firstly, make sure that you're asking for the object by the right name.
Object descriptors are case-sensitive, so asking for 'sysuptime' will
not be recognised, but 'sysUpTime' will.
Alternatively, the object may be defined in a MIB that hasn't been
loaded. Try loading in all the MIB files:
snmpget -m ALL -v1 -c public localhost sysUpTime.0
or specify the name of the appropriate MIB explicitly:
snmpget -v1 -c public myhost SNMPv2-MIB::sysUpTime.0
Note that this uses the name of the *module*, not the name of the file.
However, if 'snmpwalk' displays the object by name, this is unlikely to
be the cause, and you should look closely at the exact object name you
are using. In particular, see the next entry.
Why do I get "noSuchName" when asking for "sysUpTime" (or similar)?